This invention relates generally to frequency locked loops of the type employed in frequency synthesis circuitry commonly used in calculators and computers to provide a high frequency CPU clock based on a low frequency clock on a chip. More particularly, the present invention is directed to a frequency detector employed in such a frequency locked loop.
One known type of device employed to detect frequency is actually a phase detector whose output is a periodic function of the phase difference of signals applied at its inputs other than the desired output frequency. This device is prone to locking onto a frequency other than the desired output frequency during startup as the result of wide tolerances in on-chip component values and the fact that this type of detector utilizes only phase information in operation. Another type of prior art device for detecting frequency is a phase and frequency detector that is not periodic in the phase plane so it can only lock onto the desired frequency. However, its gain is very high, which may result in the voltage controlled oscillator following it being driven to a very high frequency immediately after turn on of the circuit. In order to eliminate this problem, the low pass filter employed in the frequency synthesis circuit requires a very long time constant, which is quite difficult to realize using on-chip components.
It is therefore an object of the present invention to provide a frequency detector that reduces the problems inherent in prior art detectors which either lock onto an undesired frequency or have such high gain that they cause a significant frequency overshoot on turn-on. This object is accomplished in accordance with the illustrated preferred embodiment of the invention by reducing the amount of phase information in the detection process and instead focusing on frequency difference information. In addition, a decision as to the frequency relationship between two input signals to the frequency detector is made only after the detection of a complete period of the highest frequency input signal.